The present invention relates to an exhaust device of an engine.
To stimulate a catalytic reaction when starting a turbocharged engine, it is known to open a wastegate valve to introduce a flow of high-temperature exhaust gas into a catalyst casing.
If a variable geometry turbocharger is adopted, since this type of turbocharger is not provided with a wastegate valve, a different approach is required for the stimulation of the catalytic reaction.
One example of a different approach is to prevent a temperature drop of the exhaust gas between a discharge from combustion chambers and an introduction into the catalyst casing.
Here, when the exhaust gas passes through exhaust ports formed in a cylinder head, due to a temperature difference between the exhaust gas and a coolant in a coolant passage formed around the exhaust ports, the heat of the exhaust gas is absorbed by the cylinder head structure and the coolant. To prevent this absorption, for example, JP2012-145065A discloses an air-cooled exhaust manifold formed as a separate body from the cylinder head and located downstream of the exhaust ports.
With the structure disclosed in JP2012-145065A, since the cylinder head has a lower temperature than the exhaust manifold due to the coolant, based on the heat transfer principles in structures, the heat may be transferred from the exhaust manifold toward the cylinder head and the exhaust gas temperature may decrease, which leaves room for further improvement.